If you do the math on solar panels on cars you realize how utterly silly the idea is from an economic point of view.

So here goes. With 100% efficient solar cells you get 1 kw per sq meter if you are aimed directly at the sun with all optimum conditions (clear sky, local noon, etc.) Now 1) Estimate how many sq meters you can arrange on a car that is street legal and safe to drive. Perhaps one. Maybe 1.5 for a bigger car; 2) how many hours per day will you get this? 3) what is the realistic efficiency of solar cells. Combine all these factors and you get how many kw-hrs you could get under all ideal conditions. I'll use 1 * 5 * .15 = .75 kw-hrs per day. I'll even stretch it to 1 kw-hr/day. In a year you'd be lucky to get 250 kw-hrs due to rainy/cloudy days. (And in most small and low drag cars you'd be lucky to get .5 sq meters so the realistic number is more like 100 kw-hrs per year)

Is this worth the cost and trouble? 250 kw-hrs is worth about $25 from the grid. If a car lasted 20 years this would be $500 of electricity.

Now how far could you go on this power? A car like the Volt or the Prius needs about 200-250 watt-hrs per mile. So you might go 1/2 mile to 1 mile extra per day, maximum on sunny days. Note that this is the best you are going to do unless you get better solar cells...so may the maximum it could ever be is twice that. In 10 years, 150K miles you might go 2000 miles from this. For these cars at 50 mpg, that is saving 40 gallons of gas...maybe it is even 50 or 100. At $3/gal that is somewhere around $120 - $300.

And we haven't even begun to subtract some things. For example the extra weight of the solar cells, can they be designed to have as low of a drag coefficient. How much energy does it take to make them? Some get totaled in car crashes, etc. Sometimes you park in the shade, etc etc.

Conclusion: It makes no economic sense. It makes no energy saving sense. If you want to buy solar cells, buy them and mount them in a permanent location so that there are always generating electricity for their entire useful life...don't drive them around in and out of the sun risking getting them smashed.